1. Field of the Invention
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The present invention relates to geophysical instrumentation systems, and more particularly, to instrumentation systems for use with surface energy sources to perform digital stacking operations on seismic signals produced from repeated applications or impulses of seismic energy applied on or near the ground surface. Throughout this disclosure the word impulse is intended to refer to the seismic energy emission from any energy source, impulsive or otherwise. Surface energy sources, such as Vibroseis, Dinoseis, etc., require stacking techniques to improve the signal-to-noise ratio to each seismic response by attenuating certain noise bandwidths which are generated by the source and contained in the signals outputted by the geophone arrays.
The signals outputted by the geophone arrays, in response to a surface energy impulse, have a very low signal-to-noise ratio because only a small part of the total energy applied to the ground is transmitted to sub-surface interfaces and reflected back to the surface. Most of each impulses energy appears as noise which propogates on or near the surface. By summing the seismic signals from repeating applications of surface energy impulses, organized to form an array, the coherent energy reflected from sub-surface strata is enhanced by the additive process, while the spatial filtering effect of the source array prevents the noise level generated by the source from increasing at the same rate.
2. Description of the Prior Art
In the prior art digital field stacking systems, a composite response to a number of energy impulse shots was obtained by summing the seismic responses from each impulse, with the surface energy source or sources stationary or moving a fixed distance between impulses. After a preselected number of impulses have been summed, usually spaced a fixed number of intervals between stations locations, the composite response is transferred to magnetic tape as a seismic record. Usually, the improvement in the signal-to-noise ratio for prior art stacking systems would be a result of the stacking process itself and, if used, the weighted geophone arrays which are producing the seismic signals at each of the stations. However, even when using these prior art systems, the source array response (the effective amplitude profile of the energy injected into the ground at each energy impulse shot point) could be varied somewhat.
In order to obtain a composite response to a variable source array in the field using the prior art stacking systems, one of two methods had to be employed. First, the surface energy source could make repeated energy impulses at each impulse point, thereby weighting the responses to that impulse point an interger number corresponding to the number of impulses. This method of shooting was only capable of synthesizing an interger-weighted variable source array. Second, the same type of variable source array, interger-weighted, could be obtained by employing an integer number of sources at each impulse point. The interger weighting factor for each impulse point would correspond to the number of energy sources used.
Although integer weighted, variable source arrays produced by the shooting methods just described provide acceptable attenuation over an acceptable bandwidth of the surface-travelling source-generated noise, they had numerous disadvantages. The first method described required an excessive number of shots to achieve the weighted composite response, and for the second method, an excessive number of energy sources are required. Both methods are slow and expensive. Additionally, the variable source array was restricted to interger weighting coefficients which limit any flexibility in the array response, or permit an optimum source array to be obtained. Although recent advances in geophone technology have enabled optimization of the geophone array response by the use of fractional weighting, it has been impossible to optimize, in the field, the complete array system (source plus geophones) due to the limitation of using integrally-weighted source arrays.
Further, in most areas, the surface noise velocity increases with distance from the source. An array length designed for maximum offset distance is longer than needed for the velocities observed at the short distances. This results in the unnecessary "smear" of wide angle shallow reflections, thus reducing the temporal resolution that is possible. An array length designed for short distances is too short for longer distances, and provides inadequate attenuation of the horizontally surface propagating noise.
U.S. Pat. No. 3,569,922 discloses a method for synthesizing a variable source array which is not limited to interger weighting. This prior art method provides for the generation of a variable source array in the laboratory, rather than in the field. A conventional seismic digital field recording system can be employed to record each and every seismic response to each and every energy impulse. The individually recorded seismic records are transported to the data reduction facility for playback. During playback, a weighting coefficient is applied to the digital samples prior to the normal stacking operations.
This prior art system has the advantage that it allows for variable source arrays to be generated from any desired weighting coefficients, rather than being restricted to intergers. However, in order to achieve this result, an excessive number of seismic records must be taken in the field. The efficiency of the field stacking technique is lost, as well as requiring a large number of field tapes to be recorded and handled, resulting in a slow turn around time from field shooting to data reduction. Additionally, there is no real time control over the quality of the composite response. This results because the stacked composite response must wait until laboratory playback before learning that changes in the shooting parameters should be implemented to obtain improvements in the composite responses. This could require that the area would have to be resurveyed.
For each of the above-described prior art systems, the source array synthesized for each of the offset distances is the same. That is, each geophone station would respond to the same source array as every other station. Therefore, the spatial density (point to point mapping) for the subsurface strata would be the same for all the traces.
To summarize, the arrays of geophones and energy impulse points are used as spatial filters for the attenuation of horizontally propagated source generated noise. In the prior art seismic field stacking systems, it has been impractical to do any of the following, in the field, during normal acquisition of seismic data from a seismic spread; (1) change the source array length with the offset distance; (2) change the source array weighting of the spatial array impulses as a function of the offset distance; (3) change the spatial density of the seismic data as a function of the offset distance; and (4) implement fractionally-weighted optimized source arrays. However, the present invention is able to achieve these results by recording, simultaneously, in a single seismic spread, data of high spatial resolution and data of normal spatial resolution. The present invention is able to modify the source array lengths at different distances to conform to the horizontal velocities of the source generated surface noise at those distances; to generate, in the field, two sets of data representing the data from two differently weighted source arrays from one set of impulses with no less in surveying time; and to implement source-geophone array combinations offering greater and more constant attentuation than has here-to-fore been possible.
Therefore, it would be advantageous to have a digital field stacking system which could synthesize variable source arrays having any desired response over a wide dynamic range, in the field, using weighting coefficients applied to the digital samples of the seismic responses prior to summation. It would also be advantageous to provide a digital field stacking system which could vary the subsurface mapping for any seismic trace by synthesizing variable source arrays whose source array spacing could be shortened without changing the shooting of the surface energy sources from their normal pattern.